I think the reason is that Raylor has been figured out as the weakest link by the opponents. The last match even though they played Nasri on the left, their actual threat comes from Milner and Micah Richards on the right, keep attacking Raylor and Jonas. In contrast, Simpson and Sammy didn't get bombed by Nasri and Clichy. Man City intentionally did that, and proved to be correct. Very likely Man Utd and Chelsea would do the same and so we have to make a change.
Possibly after we replaced Raylor with Santon, it's Simpson's turn of mistakes and errors. A new side back is a must at January, no matter right or left.